From c054ee9c9f4a802c413506d005d5841979342b31 Mon Sep 17 00:00:00 2001 From: Brandon Rhodes Date: Sat, 31 Aug 2024 14:18:37 -0400 Subject: [PATCH] =?UTF-8?q?Document=20that=20=E2=80=98SGP4=E2=80=99=20uses?= =?UTF-8?q?=20SDP4=20for=20deep=20space=20sats?= MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit --- sgp4/__init__.py | 16 ++++++++++++++-- 1 file changed, 14 insertions(+), 2 deletions(-) diff --git a/sgp4/__init__.py b/sgp4/__init__.py index 4c874c6..96a5fb5 100644 --- a/sgp4/__init__.py +++ b/sgp4/__init__.py @@ -13,9 +13,21 @@ .. _Fundamentals of Astrodynamics and Applications: https://celestrak.org/software/vallado-sw.php +What we today call ‘SGP4’ in fact merges together what in the 1970s were +originally two separate satellite propagation routines, but which are +now selected automatically: + +* SGP4 — the ‘Simplified General Perturbations' model is used for + satellites close enough to Earth that their orbit takes less than + 225 minutes (3 hours 45 minutes) to complete. + +* SDP4 — the ‘Simplified Deep Space Perturbations' model is used for + satellites farther from Earth, which take 225 minutes or longer to + compete an orbit. + If your machine can’t install or compile the C++ code, then this package -falls back to using a slower pure-Python implementation of SGP4. Tests -make sure that its positions **agree to within 0.1 mm** with the +falls back to using a slower pure-Python implementation of the library. +Tests make sure that its positions **agree to within 0.1 mm** with the standard version of the algorithm — an error far less than the 1–3 km/day by which satellites themselves deviate from the ideal orbits described in TLE files.